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Abstract:

A blade for a gas turbine includes an airfoil extending in a longitudinal
direction and extending transversely to the longitudinal direction
between a leading edge and a trailing edge. The airfoil has a pressure
side, a suction side, and a slot-like cooling medium outlet extending
along the trailing edge. The cooling medium outlet is configured to
discharge cooling medium supplied from an inner space of the blade. A
platform extends transversely to the longitudinal direction. An end of
the airfoil merges into an underside of the platform and has a transition
from the airfoil to the platform at the trailing edge of the airfoil that
increases in thickness in a direction toward the underside of the
platform. The cooling medium outlet extends into the platform so as to
reduce an operating temperature in a region of the transition from the
blade airfoil to the platform.

Claims:

1. A blade for a gas turbine, the blade comprising: an airfoil extending
in a longitudinal direction and extending transversely to the
longitudinal direction between a leading edge and a trailing edge, the
airfoil having a pressure side, a suction side, and a slot-like cooling
medium outlet extending along the trailing edge, the outlet configured to
discharge cooling medium supplied from an inner space of the blade; a
platform extending transversely to the longitudinal direction, a first
end of the airfoil merging into an underside of the platform and having a
transition from the airfoil to the platform at the trailing edge of the
airfoil that increases in thickness in a direction toward the underside
of the platform, and wherein the cooling medium outlet extends into the
platform so as to reduce an operating temperature in a region of the
transition from the blade airfoil to the platform.

2. The blade as recited in claim 1, wherein the transition from the blade
airfoil to the platform has a thickness profile having a substantially
exponential shape corresponding to at least one of an inverted rampant
pyramid, an inverted truncated pyramid, and an inverted virtual pyramid.

3. The blade as recited in claim 1, wherein the transition from the blade
airfoil to the platform has an approximately elliptical profile.

4. The blade as recited in claim 1, wherein the transition from the blade
airfoil to the platform extends to an edge of the platform.

5. The blade as recited in claim 2, wherein the transition from the blade
airfoil to the platform extends to an edge of the platform.

6. The blade as recited in claim 3, wherein the transition from the blade
airfoil to the platform extends to an edge of the platform.

7. The blade as recited in claim 1, wherein the cooling medium outlet is
formed between a pressure-side wall of the blade airfoil and a
suction-side wall of the blade airfoil, and wherein the transition from
the blade airfoil to the platform along the pressure-side wall has a
curvilinear edge profile such that a wall thickness of the pressure-side
wall in a region of the transition is approximately equal to a wall
thickness in a remaining region of the blade airfoil.

8. The blade as recited in claim 7, wherein the transition from the blade
airfoil to the platform extends to an edge of the platform.

Description:

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to International Patent
Application No. PCT/EP2009/062213, filed Sep. 21, 2009, which claims
priority from Swiss Patent Application No. 01548/08, filed Sep. 30, 2008,
each of which are incorporated by reference herein in their entirety. The
International Application was published as WO2010/037659 on Apr. 8, 2010.

FIELD

[0002] The present invention relates to the field of gas turbines, and
particularly relates to a blade for a gas turbine.

BACKGROUND

[0003] The requirements for increasing the efficiency of gas turbines
leads to the thickness at the trailing edges of the blade airfoils of the
blades which are fitted in the gas turbines having to be continuously
reduced. This results in a geometry of the blade as is exemplarily shown
in cross section in FIG. 1. The blade 10 of FIG. 1 extends in the manner
of an airfoil profile transversely to its longitudinal direction between
a rounded leading edge 15 and a comparatively sharply tapering trailing
edge 16. The blade 10 has a (concave) pressure side 13 and a (convex)
suction side 14 with corresponding walls 13' and 14'. A gaseous cooling
medium or coolant is fed in the hollow inner space 17 and discharged into
the environment inter alia through a cooling medium outlet which is
formed at the trailing edge 16. A particularly sharply tapering, slender
trailing edge 16 in this case is achieved by the cooling medium outlet 18
being arranged entirely on the pressure side 13 of the blade 10, and by
the two walls 13' and 14' being constructed especially thin in the region
of the trailing edge 16.

[0004] If, as is shown in the perspective view of FIG. 2, the blade
airfoil 11 at the end of its extent merges in the longitudinal direction
into a platform 12 which lies transversely to the longitudinal direction
and is delimited by this platform 12, the transition of the blade airfoil
11 to this platform 12 in the region of the trailing edge 16 represents a
typical factor which limits the service life of a gas turbine component
because it is exposed to superposition of high thermal stress which is
brought about by the thermo-mechanical mismatch between platform 12 and
blade airfoil 11, and to mechanical stress peaks which are brought about
by loading of the blades as a result of the gas flow. Reducing the
thickness of the trailing edge 16 causes an increase of the stress in
this critical region so that when designing the blade measures have to be
considered in order to achieve and to ensure a sufficiently long service
life.

SUMMARY

[0005] An aspect of the present invention is to further develop a blade
for a gas turbine so that despite a low thickness at the trailing edge of
the blade airfoil a satisfactory service life is achieved.

[0006] In an embodiment, the present invention provides a blade for a gas
turbine including an airfoil extending in a longitudinal direction and
extending transversely to the longitudinal direction between a leading
edge and a trailing edge. The airfoil has a pressure side, a suction
side, and a slot-like cooling medium outlet extending along the trailing
edge. The cooling medium outlet is configured to discharge cooling medium
supplied from an inner space of the blade. A platform extends
transversely to the longitudinal direction. An end of the airfoil merges
into an underside of the platform and has a transition from the airfoil
to the platform at the trailing edge of the airfoil that increases in
thickness in a direction toward the underside of the platform. The
cooling medium outlet extends into the platform so as to reduce an
operating temperature in a region of the transition from the blade
airfoil to the platform.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Exemplary embodiments of the present invention are described in
more detail below, with reference to the drawings. Some non-essential
elements of the invention have been omitted. Like elements are provided
with the same designations in the different figures. In the drawings:

[0008] FIG. 1 shows a simplified cross section through an gas turbine
blade with a narrow trailing edge and a cooling medium outlet;

[0009]FIG. 2 shows a sharp transition between blade airfoil and platform
in a blade such as that shown in FIG. 1; and

[0010]FIG. 3 shows a low-stress transition between blade airfoil and
platform according to an exemplary embodiment of the invention.

DETAILED DESCRIPTION

[0011] In an embodiment, the present invention provides a transition from
the blade airfoil to the platform at the trailing edge that has a
transition thickness profile in which the thickness increases above
average the closer it gets to the underside of the platform, and that the
cooling medium outlet is extended right into the platform for reducing
the temperature in the region of the transition from the blade airfoil to
the platform. As a result of increasing the thickness of the trailing
edge towards the platform the mechanical stress in the transition region
is reliably reduced. Extending the cooling medium outlet right into the
platform leads to improved cooling there so that thermally induced
stresses are also significantly reduced.

[0012] In one embodiment, the transition thickness profile has an
essentially exponential shape which resembles an inverted, very slender,
truncated pyramid or an inverted virtual pyramid. As a result of this an
especially "smooth" transition between trailing edge and platform is
achieved.

[0013] In another embodiment, the transition from the blade airfoil to the
platform has an approximately elliptical transition border profile which
also reduces stresses.

[0014] Furthermore, it is advantageous if according to another development
the trailing edge at the transition from the blade airfoil to the
platform is extended up to the edge of the platform.

[0015] In an other embodiment, the cooling medium outlet is formed between
a pressure-side wall of the blade airfoil and a suction-side wall of the
blade airfoil, and in that the pressure-side wall has a curvilinear
transition edge profile in the transition from the blade airfoil to the
platform in such a way that the wall thickness of the pressure-side wall
in the region of the transition from the blade airfoil to the platform is
approximately equal to the wall thickness in the remaining region of the
blade airfoil.

[0016] In FIG. 3, a blade 20 for a gas turbine with a low-stress
transition between blade airfoil 11 and platform 12 according to an
exemplary embodiment of the invention is reproduced. The blade 20 of the
exemplary embodiment comprises a blade airfoil 11 which extends in a
longitudinal direction and in the manner of a wing extends transversely
to the longitudinal direction between a leading edge 15 and a trailing
edge 16, and has a pressure side 13 and a suction side 14. At the upper
(or lower) end the blade airfoil 11 merges into a platform 12 which lies
transversely to the longitudinal direction and projects laterally across
the blade cross section. A slot-like cooling medium outlet 18 which
extends along the trailing edge 16 is provided at the trailing edge 16 of
the blade airfoil 11, through which a cooling medium, for example cooling
air, which is fed via the (hollow) inner space 17 of the blade 20, is
discharged. The trailing edge 16 with its thin walls 13' and 14' is very
narrow in construction. In order to reduce the thermal stresses at the
transition between the narrow trailing edge 16 and the solid platform 12,
according to embodiments of the invention the transition has a transition
thickness profile 21 in which the thickness D increases above average the
closer it gets to the underside 12' of the platform 12. At the same time,
the cooling medium outlet 18 is extended (extension 19) right into the
platform 12 for reducing the local temperature in the region of the
transition from the blade airfoil 11 to the platform 12.

[0017] The transition thickness profile 21 has an essentially exponential
shape and as a result resembles an inverted rampant pyramid. It is
especially favorable in this case with regard to the stress distribution
if the transition from the blade airfoil 11 to the platform 12 has an
approximately elliptical transition border profile 22. While in the case
of the conventional blade according to FIG. 2 the trailing edge 16 of the
blade airfoil 11 terminates inside the platform 12 and does not extend as
far as the boundary of the platform 12, in the case of the exemplary
embodiment of FIG. 3 the trailing edge 16 at the transition from the
blade airfoil 11 to the platform 12 is extended up to the edge 12'' of
the platform 12.

[0018] As is to be seen in FIG. 3, the cooling medium outlet 18 is
delimited by the pressure-side wall 13' and the suction-side wall 14' of
the blade airfoil 11. The pressure-side wall 13' in this case has a
curvilinear transition edge profile 23 in the transition from the blade
airfoil 11 to the platform 12 in such a way that the wall thickness of
the pressure-side wall 13' in the region of the transition from the blade
airfoil 11 to the platform 12 is approximately equal to the wall
thickness in the remaining region of the blade airfoil 11.

[0019] In all, an appreciable improvement of the service life at the
transition between the blade-airfoil trailing edge and the platform of a
gas turbine blade is achieved by the invention as a result of the
following measures: [0020] (1) Extending the cooling medium outlet
(cooling slot) into the platform in order to reduce the metal temperature
in the critical region by feeding cooling medium, wherein convective
cooling of the walls on both sides takes place. [0021] (2) Relocating the
blade-airfoil trailing edge to the boundary of the platform in order to
lower stress and to make the design of the blade independent of
deviations in the radial position of the cast core. [0022] (3)
Introducing a transition thickness profile of a rampant pyramid type by
increasing the height and introducing a special elliptical contour of the
fillet at the transition between blade airfoil and platform in the region
of the trailing edge. [0023] (4) Introducing a specially curvilinear
transition edge profile towards the pressure side of the blade airfoil on
the fillet at the transition between blade airfoil and platform in the
region of the trailing edge in order to achieve a wall thickness in the
transition region which corresponds to the wall thickness of the blade
airfoil, as a result of which stress and metal temperature is reduced and
service life at the transition is increased.